[ Pobierz całość w formacie PDF ]
.Shortly after,however, these chlorinated adhesion promoters were substituted with light-graytwo-component (2K) primers.These primers, in addition to providing coverageof black TPO materials, gave better performance in difficult and severe tests thatwere being developed and implemented by OEM customers (7).The principle ofthese 2K primers is to combine a resin that is able to be crosslinked with aCPO.Both act synergistically to enhance adhesion on TPO substrates.The Automotive Plastic Coatings in Europe 327FIG.4 Exhaust gas absorption unit for fluorine pretreatment of plastic parts.(Courtesy of Fluor Technik System GmbH, Lauterbach, Germany.) 328 Gruner and ReinhartTABLE 5 Chemical Reactions in Fluorine Treatment=CH2 + F2 > =CHF + HF (Surface treatment step)2HF + CaCO3 > CaF2 + H2O + CO2 (Absorption of exhaust gas)1DF2 + CaCO3 > CaF2 + 2O2 + CO2 (Absorption of exhaust gas)crosslinking mechanism involves the reaction of either amine or hydroxyl func-tional groups with epoxy or isocyanate hardeners, respectively.For electrically conductive primers, conductive carbon black and graphiteare used as the conductive pigments.To give at least a dark gray color, just theamount of titanium dioxide white pigment is used to maintain a surface resistanceof about 20 k-ohm/cm.Unfortunately, the result leaves something to be desired.Therefore, alternative conductive paint ingredients have been developed and com-mercialized and today medium to light gray primers are available using conductivepigments based on antimony-doted stannous oxide layers (16).However, the veryhigh cost of these conductive pigments has restricted their growth dramatically.When coating TPO in Europe, usually a combination of some type ofpretreatment plus 2K adhesion primers are used.The pretreatment provides opti-mum adhesion to even pass hot-water jet tests in which scribed painted partsare exposed to 100 percent relative humidity for ten days at 40°C before beingFIG.5 Process control of fluorine treatment.Pressure Unit 1000mbar = 14.7 psi,Stage I: Evacuation, Stage II: Inlet Fluorine, Stage III: Treatment, and Stages IV, V,VI: Flushing.(Courtesy of: Fluor Technik GmbH, Lauterbach, Germany.) Automotive Plastic Coatings in Europe 329blasted with hot water.The role of the primer is to provide adhesion in anyareas that may be insufficiently flamed due to wrong positioning of skids to therobots.Failures caused by this, for example, on lamp openings and grille areasof bumpers can be prevented by this dual belt and suspender strategy.Significant progress has been made to develop primerless coatings forTPO bumpers using additives that make the basecoat adhere directly to theplastic surface (17,18) without the need for chlorinated adhesion promoters.Using statical mixing equipment, the additives are mixed in the basecoats forpaint being applied through the first robot.A second spray of basecoat is carriedout with the conventional basecoat not containing the additives.This formula-tion technique and application process offers a considerable cost-saving opportu-nity because a layer of chlorinated primer is eliminated.An unexpected benefitof this additive approach was a smoother topcoat appearance (less orange peel)due to superior basecoat wetting of the TPO substrate.Waterbased primers have been introduced to reduce solvent output fromTPO component coatings in Europe.As these do not contain efficient adhesion-promoting ingredients for TPO substrates, the result is strictly dependent on thechemical pretreatment.Commercial experience with these waterbased productshas been limited to polypropylene surfaces that have been perfectly flamed andthus the scope of this technology at present is limited to relatively simply shapedparts.New experimental primers with inherent adhesion to TPO are under evalu-ation and will allow for substitution of their solventbased counterparts in thevery near future.Some degree of flexibility must be built into these primers because aftertopcoating with conventional basecoat and clearcoat, the flexible clearcoat sys-tems have to pass the low-temperature impact resistance required for automotivebumper fascias and other components in the impact area of the vehicle [ Pobierz caÅ‚ość w formacie PDF ]